1. Field of the Invention
The present invention relates generally to a hydraulic control apparatus for a hydraulically-operated automatic transmission of a motor vehicle, and more particularly to such a hydraulic control apparatus for regulating the line pressure used to shift the transmission.
2. Discussion of the Prior Art
For a motor vehicle, there is widely known an automatic transmission which uses a plurality of hydraulically operated frictional coupling devices such as multiple-disk clutches and band brakes. A hydraulic circuit including a pressure source which generates a hydraulic pressure is provided to selectively operate the frictional coupling devices and thereby establish a suitable one of a plurality of operating positions (speed or gear positions) of the transmission. If the line pressure to be applied to the frictional coupling devices is excessively low, the frictional coupling devices tend to easily slip, leading to seizure or rapid wearing of the coupling members of the devices. If the line pressure is excessively high, the frictional coupling devices tend to undergo abrupt engaging actions, resulting in undesirable shifting of the transmission with a large shifting shock. The excessively high line pressure requires a relatively long operating time of a hydraulic pump of the hydraulic pressure source, which means a large amount of energy consumption or loss. In the light of these drawbacks, the line pressure for the transmission is generally regulated so as to be at a required minimum level, depending upon the amount of torque that should be transmitted through the transmission, namely, depending upon the angle of opening of a throttle valve which represents the current load of the vehicle engine whose output is transmitted to the drive wheels of the vehicle through the transmission. An example of such hydraulic control apparatus adapted to regulate the line pressure as described above is disclosed in JP-A-61-248949 (laid-open publication of unexamined Japanese Patent Application).
The present applicants recognized a drawback in the known hydraulic control apparatus adapted to regulate the line pressure depending upon the throttle opening of the engine. Namely, the throttle opening which has been used as a parameter .representative of the engine load does not actually reflect the engine load with high precision, on motor vehicles of modern vintage which are equipped with various adjusting devices or mechanisms for improving fuel economy of the engine or optimizing output characteristics of the engine under varying running condition of the vehicle. Those adjusting devices or mechanisms include: mechanism for changing the operating timing of the intake and exhaust valves of the engine; mechanism for changing the idling speed of the engine; and exhaust gas recirculating valve (EGR valve ) for recirculating the exhaust emission back to the air intake system. The engine load may change due to a change in the intake air quantity, even with the same throttle opening, depending upon the atmospheric pressure which varies with the altitude.
On the other hand, the line pressure can be suitably regulated without an influence of the adjusting devices or mechanisms and the altitude, when the line pressure regulation is based on the intake air quantity of the engine which is actually detected. However, a change in the intake air quantity is delayed with respect to a change in the throttle opening, when the vehicle is accelerated or otherwise in a transient running state. This delay may cause the line pressure to be temporarily lower than required. When the torque to be transmitted through the transmission ( transmission torque) is increased during acceleration of the vehicle, the line pressure should be increased before the increase of the transmission torque. That is, an increase in the line pressure after an increase in the actual engine load (transmission torque) does not assure optimum line pressure for transmitting the increased torque.